Let's say I've got a friend (I do!) and this particular friend wants to have sex with a mentally unstable (but not dangerous) woman in a committed relationship. He persuades her to do so, and she betrays her boyfriend to have sex with this friend. This friend has no intention of an ongoing relationship with the woman - he just wants sex, and tells her so all along.

Obviously she's morally (or ethically, if you prefer) culpable for betraying a trust. Is the friend who had sex with her morally or ethically culpable for a wrong act, and if so - what?